Molecular genetics of mammalian differentiation and oncogenesis studied in transgenic mice. The project comprises several approaches, some of them reflecting collaborations with other laboratories. 1. Oncogenesis of the embryonic lens. The embryonic lens vesicle undergoes a well defined fiber differentiation program, starting at mid gestation, that ultimately results in the formation of the translucent adult lens structure. We have targeted the expression of the large tumor antigens of the DNA viruses polyoma and SV40 to the developing lens in an effort to understand how these oncogene products interfere with the orderly process of lens differentiation. The polyoma T antigen immortalizes but does not transform lens cells. Its action is partly abrogated by basic fibroblast growth factor and by insulin, factors that have been implicated in normal lens differentiation. By contrast, the SV40 T antigen causes lens tumors. The rate of tumor progression differs between individual lines of transgenic mice and appears to be linked to the state of differentiation attained in the embryonic lens at the time of T antigen accumulation. The results obtained with the two viral tumor antigens thus underscore the importance of the differentiating cell as the site where oncogene products compete with regulators of cell growth for the control of the cell cycle and where the fate of the organism is at stake. The availability of immortalized lens cells has enabled us to clone and characterize a gene that specifies a factor implicated in the regulation of lens-specific crystallin expression. 2. Gene targeting by homologous recombination. The purpose of this aspect of the project is to alter resident chromosomal genes of the mouse and to characterize the resulting mutant phenotypes. Following a technically demanding sequence of experimental steps, a wild type gene sequence is replaced in a pluripotent embryonic stem (ES) cell by inserting a mutated sequence in its place. The mutant ES cell is injected into a blastocyst which is transferred into a foster mother in an effort to obtain germ line chimeras that can propagate the mutant trait. We have established this technique in the laboratory and are presently targeting the c-fos proto-oncogene. In addition, we have joined collaborative efforts aimed at generating a mouse model of Gaucher's disease, a human disorder based on defined mutations of the glucocerebrosidase gene.